Spread spectrum variable data rate coherent phase-shift-keyed communication links offer a powerful trade-off between imagery data rate and jam resistance/low probability of detection (JR/LPD) performance. However, the implementation of such a data link requires considerations of waveform design and synchronization concepts beyond those required for a fixed data rate system. Almost invariably, analyses and studies indicate that the preferred data waveform is one which incorporates a coherent residual carrier component which is used for detection and tracking in the receiver. Such a waveform obviates the multiplicity of data rate dependent filters normally used in receivers which recover the carrier phase by non-linear methods (e.g., nth-power and Costas type designs).
This invention employs coherent BPSK for both pseudorandom noise (PN) and data modulation because of its simplicity and low risk implementation at high rates. The long term balance of "one's" and "zero's" in conventional BPSK modulation ensures the (theoretical) absence of a discrete carrier component in the RF waveform; hence the term "suppressed" carrier. Coherent demodulation of this waveform (suppressed carrier), however, requires a carrier, and the carrier is typically derived from the waveform by non-linear operations in the receiver. Whether the non-linear operation is at IF (e.g. squaring loop) or baseband (e.g., Costas loop) an inevitable loss in signal-to-noise ratio (SNR) results. Under low input predetection SNR's, small signal suppression effects cause system performance to degrade faster than the input SNR; in the cases of the carrier and N code tracking loops, undesirable parameter changes (loop gain, damping and bandwidth) result. In order to minimize precipitous degradation in these loops, data rate dependent filters are required to maintain the predetection SNR as high as possible. In a multi-rate system, a fully optimized suppressed carrier receiver design would require as many different filter bandwidths (whether IF or baseband) as data rates and possibly several different IF frequencies to support all of the bandwidths.
The penalty for not optimizing the performance for each data rate is, as implied above, non-graceful degradation. Loop bandwidths adapt to the rapidly reducing input SNR, quickly becoming too narrow to track the carrier or N code frequency dynamics, and tracking thresholds are reached just a few dB below the operating point for low error rate data recovery.
The motivation for a residual carrier waveform design is the need to avoid the non-linearity losses suffered in multi-bit rate suppressed carrier designs. If the transmitted waveform contained within its structure is a relatively low power coherent carrier, then this carrier would be available immediately in the receiver for detection and tracking; non-linear processes and the attendant performance degradation would be avoided. Such a residual carrier would not be detectable as a discrete line spectrum in the transmitted waveform because it, too, would be spread by the spreading waveform.
If, instead of a long term balance of "one's" and zero's" in the data stream, an unbalance is forced, then the resulting waveform at RF would contain a residual carrier. This is the concept behind a shared energy signal (SES) waveform. However, the unbalance is not created by a simple stuffing of extra "one's" or "zero's", as this would be inefficient, but rather by a unique coding scheme which provides the desired unbalance. The coding is 4-ary, providing a residual carrier 6 dB down.